Oleg Izhvanov

Spark Plasma Sintering of Commercial Zirconium Carbide Powders: Densification Behavior and Mechanical Properties

Commercial zirconium carbide (ZrC) powder is consolidated by Spark Plasma Sintering (SPS). Processing temperatures range from 1650 to 2100 °C. Specimens with various density levels are obtained when performing single-die SPS at different temperatures. Besides the single-die tooling setup, a double-die tooling setup is employed to largely increase the actual applied pressure to achieve higher densification in a shorter processing time. In order to describe the densification mechanism of ZrC powder under SPS conditions, a power-law creep constitutive equation is utilized, whose coefficients are determined by the inverse regression of the obtained experimental data. The densification of the selected ZrC powder is shown to be likely associated with grain boundary sliding and dislocation glide controlled creep. Transverse rupture strength and microhardness of sintered specimens are measured to be up to 380 MPa and 24 GPa, respectively. Mechanical properties are correlated with specimens’ average grain size and relative density to elucidate the co-factor dependencies.

Zirconium Carbide Produced by Spark Plasma Sintering and Hot Pressing: Densification Kinetics, Grain Growth, and Thermal Properties

Spark plasma sintering (SPS) has been employed to consolidate a micron-sized zirconium carbide (ZrC) powder. ZrC pellets with a variety of relative densities are obtained under different processing parameters. The densification kinetics of ZrC powders subjected to conventional hot pressing and SPS are comparatively studied by applying similar heating and loading profiles. Due to the lack of electric current assistance, the conventional hot pressing appears to impose lower strain rate sensitivity and higher activation energy values than those which correspond to the SPS processing. A finite element simulation is used to analyze the temperature evolution within the volume of ZrC specimens subjected to SPS. The control mechanism for grain growth during the final SPS stage is studied via a recently modified model, in which the grain growth rate dependence on porosity is incorporated. The constant pressure specific heat and thermal conductivity of the SPS-processed ZrC are determined to be higher than those reported for the hot-pressed ZrC and the benefits of applying SPS are indicated accordingly.

Effects of loading modes on densification efficiency of spark plasma sintering: sample study of zirconium carbide consolidation

Abstract Theoretical studies on the densification kinetics of the new spark plasma sinter-forging (SPS-forging) consolidation technique and of the regular SPS have been carried out based on the continuum theory of sintering. Both modelling and verifying experimental results indicate that the loading modes play important roles in the densification efficiency of SPS of porous ZrC specimens. Compared to regular SPS, SPS-forging is shown to be able to enhance the densification more significantly during later sintering stages. The derived analytical constitutive equations are utilised to evaluate the high-temperature creep parameters of ZrC under SPS conditions. SPS-forging and regular SPS setups are combined to form a new SPS hybrid loading mode with the purpose of reducing shape irregularity in the SPS-forged specimens. Loading control is imposed to secure the geometry as well as the densification of ZrC specimens during hybrid SPS process.

Cylindrical Inverted Multi‐Cell (CIM) Thermionic Converter for Solar Power and Propulsion Systems

Design and fabrication of a novel four cell cylindrically inverted multi‐cell (CIM) solar thermionic converter for space power applications is in progress. When heated externally, the converter (8 cm diameter and 35 cm long) is capable of producing up to 492 W of electric power. The emitters operate at 2000 K while the collectors operate at 1050 K. Key components include four polycrystalline rhenium (Re) emitters, a cesium (Cs) containment vessel, and a collector trilayer. The emitters and containment vessel are both made from Re tubing. Hot Isostatic press (HIP) processing is used to produce the collector trilayer which is made up of a niobium alloy (Nb‐1%Zr) base tube, plasma sprayed aluminum oxide insulation layer, and a niobium collector tube. After machining the Re emitters are brazed to the Nb collectors. The purpose of this paper is to describe the design and fabrication of the four cell CIM which will be tested at General Atomics (GA).

Conductively coupled multi-cell TFE with electric heating pretest ability

Problems associated with the development of a multi-cell thermionic fuel element (TFE) with ability of electric heating test are discussed. A conceptual design of such TFE with trilayer emitter stack is proposed. Trilayer emitter stack consists of a strong emitter fuel clad coated with a high temperature oxide ceramic. Emitter tungsten coatings applied to a ceramic and they separated one after another by insulated gaps. Modern materials that should be base to build this trilayer emitter are presented. Results of calculational investigations of TFE output parameters are included. Results of the preliminary test of TFE and it’s components are presented. It is shown that proposed TFE conceptual design from one side allows to provide high output parameters inherent to multi-cell design, and from other side to gain advantages of single cell TFE, such as TFE and reactor nuclear safety, reliability, work cost savings.

A single‐cell TFE mock‐up of the thermionic nuclear power system ‘‘Space‐R’’

The results of calculation and design developments of a single‐cell thermionic fuel element (TFE) for the thermionic nuclear power system (NPS) ‘‘Space‐R’’ of 40 kW power are given in this paper. Calculation characteristics of the TFE: thermal, electrical, strength, life‐time and also results of preliminary tests, are discussed.

Development and testing of conductively coupled multi-cell TFE components

The technology development and modeling results for conductively coupled multi-cell thermionic fuel element components are presented. Different design versions of the converter structural units are considered. The results of thermal test of the main converter components are presented.

Thermionic converters with planar electrodes for solar power and propulsion systems

Different versions of thermionic converters (TICs), as well as modes of operation, are considered with an emphasis on solar power and propulsion systems. Output power parameters of the TICs operating in the ignited, Knudsen and quasivacuum modes of operation are presented. Comparison of these operation modes are performed. The main components of thermionic converters: electrodes; metal-ceramic seals; and spacing systems that ensure high performance and long life are discussed.

Development of a cylindrical inverted thermionic converter for solar power systems

The design, development and fabrication of a single-cell cylindrical inverted converter are presented. The converter is a coaxial system of two electrodes, in which the outer clad heats the emitter directly and a collector is set inside the emitter. The materials and design elements of a testable prototype are described. Different design versions of structural system components are considered. The results of preliminary tests of the main system components are presented.

SPACE‐R nuclear power system TFE mock‐up SC‐320 demonstration test

As a result of its 1993–94 work, NII NPO ‘‘LUCH’’ developed a thermionic fuel element (TFE) SC‐320 intended for use as part of a nuclear thermionic reactor‐converter known as SPACE‐R designed in the US and rated at 40 kW of output electric power. This paper presents the results of the demonstration electric power tests of the SC‐320 TFE mock‐up conducted in the US at the TSET testing facility located at the University of New Mexico. The data obtained are compared to the calculated characteristics as well as the output parameters of the Topaz‐2 NPS TFEs.